1. Technical Field
This invention relates generally to vacuum fluorescent displays and more particularly to a power supply for providing DC filament voltage to a static vacuum fluorescent display.
2. Discussion of the Prior Art
Vacuum fluorescent (VF) displays are generally defined by an evacuated tube enclosing one or more phosphor-coated anodes arranged in a predetermined pattern for emitting light, a filament and a grid disposed Is between the anodes and filament. The filament is electrically heated at a relatively low voltage to generate a cloud of electrons. In small VF displays (approximately fifty segments or less) a DC filament voltage of 2 to 3 volts can be used. In contrast, the grid is maintained at a relatively high voltage to accelerate electrons onto any of the anodes which are also maintained at a relatively high voltage. The phosphor-coated anodes emit light when excited by the accelerated electrons.
A typical VF display system for use in an automobile is shown in the prior art drawing of FIG. 1. Referring to FIG. 1, conventional battery 10 is connected by ignition switch 12 to terminal 14 which, when referenced to the vehicle ground, is generally referred to as ignition voltage IGN. VF display 16 comprises a plurality of anode segments 18, filament 20, and grid 22 disposed therebetween. Each anode segment 18 may be selectively connected to ignition voltage IGN through display driver 24 which typically is coupled to a microprocessor (not shown) for controlling which segments to light. Anode segments 18 are arranged in a predetermined pattern to create symbols, numerals, characters, etc. For example, to display time in a clock, or current temperature in a climate control panel.
To protect against excess ignition voltage IGN fluctuations, voltage limiting circuitry 26 is typically interposed between the ignition voltage input terminal 14 and both display driver 24 and grid 22. A blocking diode 28 may also be included to protect against a reverse battery condition. Accordingly, power to grid 22 and anode segments 18 are referenced to a relatively high voltage close to ignition voltage IGN. In contrast, filament 20 is energized to a relatively low potential by initially stepping down ignition voltage IGN to a regulated 5 volts through resistor 30 and voltage regulator 32. Finally, the nominal filament voltage V.sub.f, typically 2 to 3 volts in automotive applications, is provided by further stepping the regulated voltage down through resistor 34.
The inventors herein have recognized several problems and disadvantages of the prior art approach discussed above. Particularly, the above approach requires the expense of a dedicated integrated circuit; voltage regulator 32 in this case. Moreover, the circuit must be designed to stay in regulation and to adequately dissipate power over the range of ignition voltage which typically fluctuates between 8 and 18 volts. Resistors 30 and 34, while helpful in regards to establishing the proper filament voltage, provide minimal assistance in dissipating power because of the necessity of keeping voltage regulator 32 in regulation over the entire range of ignition voltage IGN. Moreover, large heat sinks may be necessary to remove heat from voltage regulator 32 because of the limited amount of power which may be dissipated over resistor 30.
In some applications, it may be desirable to use voltage regulator 32 to power other circuitry in the display module in addition to providing power to the filament. However, a likely consequence of sharing regulator 32 is that it will be subjected to noise generated by the filament which may be difficult to suppress. Accordingly, a second voltage regulator 36 is typically included to support the other 5 volt requirements.
An alternative to the dedicated voltage regulator 32 shown in FIG. 1 is a pass transistor in combination with an operational amplifier. Like the approach of FIG. 1, however, this system would similarly require a dedicated integrated circuit designed to operate within regulation to accommodate the full range of ignition voltage fluctuation. Accordingly, there is a need for a low cost filament power supply with improved power dissipation.